The invention concerns a printed valuable document with at least one authentication feature in the form of a luminescent substance based on a host lattice doped with at least one rare earth metal.
The protection of valuable documents by means of luminescent substances has long been known. The use of rare earth elements in this connection has also been discussed. These have the advantage of possessing narrow band emission lines in the infrared spectral region that are particularly characteristic and can therefore be safely distinguished from the emissions of other substances. These luminescent substances on the basis of rare earth metals usually involve a host lattice doped with a rare earth metal. In order to detect this luminescent substance, it is exposed to a light source whose emission spectrum largely overlaps the excitation spectrum of the rare earth metal and therefore selectively detects the emission lines produced by the luminescent substance.
In order to enhance the emission intensity of the luminescent substance, it has also already been suggested that co-activated luminous materials be used. These contain the two rare earth metals neodymium (Nd) and ytterbium (Yb) as activators. In this case, the Nd is selectively excited by means of a GaAs diode in the 800 nm region, and it then transfers the absorbed energy with a high level of efficiency to the Yb and thus induces the emissions from the Yb.
The invention is based on the aim of making available a valuable document with a luminescent substance based on host lattices doped with rare earth metals, which are excitable in the visible spectral region and have a high level of emission intensity in the near IR spectral region.
The fulfillment of this aim is embodied in the features of the non-dependent claims. Further developments of these are the subject of the dependent claims.
The invention is based on the fundamental premise that the emission intensity can be increased if the host lattice itself has absorptive components that absorb across a broad band and transfer this energy with a high degree of efficiency to the luminescent rare earth metals. Preferably, lattices are used for this which, on the one hand, absorb in the entire visible spectral region, but which, on the other hand, are largely transparent in the near IR region. This has the advantage that strong light sources, such as halogen, xenon, arc or flash lamps can be used for excitation of the luminescent substances. At the same time, the host lattice should be optically transparent in the region of the luminescent substance""s emission bands. According to the invention, this region lies in the near infrared between 0.8 xcexcm and 1.1 xcexcm, so that there is a relatively broad non-absorptive xe2x80x9cwindowxe2x80x9d, in which the most varied of emission spectra can be implemented.
The host lattice according to the invention contains chromium as the absorptive component. The rare earth metals in this case may be ytterbium, praseodymium or neodymium. The host lattice can also have several rare earth metal dopings.
Preferably the host lattice has a garnet or perovskite structure.
The absorptive host lattice components can be partially replaced with the non-absorptive aluminium. The proportion of aluminium can be used to control the absorption and thus the brightness of the luminescent substance. Luminescent substances of this type can therefore also be used as additives for lighter printing inks.